In the implantable medical device field, a medical device, configured to perform a desired medical function, is implanted in the living tissue of a patient so that a desired function may be carried out as needed for the benefit of the patient. Numerous examples of implantable medical devices are known in the art, ranging from implantable pacemakers, cochlear stimulators, muscle stimulators, glucose sensors, and the like.
Some implantable medical devices are configured to perform the sensing function, i.e., to sense a particular parameter, e.g., the amount of a specified substance in the blood or tissue of the patient, and to generate an electrical signal indicative of the quantity or concentration level of the substance sensed. Such electrical signal is then coupled to a suitable controller, which may or may not be implantable, and the controller responds to the sensed information in a way to enable the medical device to perform its intended function, e.g., to display and/or record the measurement of the sensed substance. An example of an implantable medical device that performs the sensing function is shown, e.g., in U.S. Pat. No. 4,671,288.
As medical devices have become more useful and numerous in recent years, there is a continual need to provide very low power sensors that may be connected to, or incorporated within, such devices so that the desired function of the device can be carried out without the expenditure of large amounts of power (which power, for an implanted device, is usually limited.)
Moreover, as the power consumption of many implantable sensors is deliberately designed to be very low, the output signal generated by the sensor (which output signal represents a measure of the parameter or substance being sensed by the sensor) becomes very small (e.g., extremely small amplitude). This small output signal must eventually be converted to a more useful output signal, e.g., an amplified signal, or a digital signal, before it can be used to control the medical device, or be displayed by the medical device. Further, ofttimes the sensor itself is located some distance from the medical device which needs the information measured by the sensor. Hence, the sensor signal must be sent to the medical device over a suitable conductor, or otherwise transmitted to the medical device. Because the sensor output signal is so small, it generally first must be amplified, or otherwise converted to a signal in a more usable format (i.e., converted to a digital signal by an analog-to-digital (A/D) converter) before it can be reliably sent or transmitted to the medical device. Unfortunately, such amplification and/or A/D conversion requires additional circuitry, located at the sensor site. Disadvantageously, this additional circuitry located at the sensor (which may be incorporated as part of the sensor, or becomes supplemental circuitry that must be used with the sensor), not only places additional power demands on the system, but it also may dramatically increase the circuit complexity size, and cost of the sensor circuitry. What is needed, therefore, is an extremely low power conversion circuit that converts the very small output signals typically obtained from implantable sensors to a signal format that facilitates the signal's subsequent transmission to and use by the medical device.
The above need is even more acute where more than one sensor must be used. More than one sensor may be needed, for example, to measure more than one substance or physiological parameter. In other instances, more than one sensor may be needed to measure or sense the same substance or physiological parameter at different locations within the patient's body. Whenever multiple sensors are implanted and are intended to be used in concert to achieve a desired medical function, there is a corresponding need to connect or couple such separate multiple sensors to a single control circuit or common control point. Hence, there is a critical need that the output signal (representing output data) from each sensor be first converted to a format that facilitates transmission of the sensor output signal over a shared data bus or communication channel without compromising the integrity of the data, and that such conversion not consume large or even moderate, amounts of power.